As the size of electronic components used in integrated circuits has shrunk and their density in circuits increased, there has been growing interest in electron beam lithography. In electron beam lithography, an electron beam is used to write patterns on an electron beam resist layer formed over a top surface of the semiconductor wafer that is the workpiece for the electron beam lithography. A particular form of electron beam lithography of special interest for the invention is described as Scattering With Angular Limiting Projection Electron Beam Lithography (“Scalpel”).
Scalpel is an electron beam lithography technique that typically has employed masks in which the mask used for control in the irradiation of the workpiece has been formed as a thin patterned coating supported on a thin membrane. Scalpel has been described in various publications.
The requirements for such a membrane-mask combination are quite demanding. Typically, it needs to be planar, relatively easy to make, and comparatively rugged. A particular problem has been the forming of the membrane that is to support the coating in which the mask pattern is formed. The membrane needs to be of relatively large area, typically at least several inches on a side, to be practical for use in large scale manufacture. It needs also to be very thin, typically no thicker than about 1000 Angstroms in thickness, to permit electrons that pass through the mask to penetrate it easily without excessive scattering or experiencing excessive loss in electron beam energy. Moreover, it also needs to remain planar with inappreciable sag in use so that it continues to support the mask uniformly over its entire area.
To insure that the membrane remains sag-free, it is generally the practice to suspend the membrane by a substrate that provides a suitable underlying support grillage, typically consisting of major struts and minor struts (ribs).
Hitherto, for forming the grillage for supporting the membrane that supports the mask coating, a layer of silicon nitrite was deposited over the top surface of a silicon wafer and then the silicon wafer was etched to leave a portion thereof to provide the supporting grillage for the nitride coating that was to serve as the membrane. However, this has proven difficult.
One problem with this approach has been the difficulty of maintaining, during the shaping of the silicon wafer, the mechanical stability of the grillage left to support nitride the membrane. The physical dimensions of commercially available silicon wafers limits formation of thin struts needed.
The present invention involves a better way to prepare a mask useful in electron beam lithography.